The overall goal of the experiments performed in this laboratory are to identify the mechanisms controlling the activity of the hypothalamus-pituitary-adrenal (HPA) axis in fetal sheep. Providing a more complete understanding of the activity of the HPA axis will be key to understanding fetal stress, homeostasis, and (in sheep and perhaps in other species) the control of parturition. In past years, we have investigated several of the physiological and endocrine mechanisms controlling the activity of the ovine fetal HPA axis. Recently, we have reported that estrogen potently stimulates the activity of the fetal HPA axis. At the end of gestation in sheep and in other species, there is an increase in the plasma concentrations of estrogen. The estrogen in fetal plasma is both unconjugated and sulfoconjugated. The present proposal is assess the efficacy of sulfoconjugated estrogens in the stimulation of the fetal HPA axis. To achieve these aims, we will perform experiments using in vivo, biochemical, and molecular techniques. All of the experiments will be based on the study of the HPA axis control in chronically-catheterized fetal sheep. The proposed experiments will test the hypothesis that estrogen receptors and steroid sulfatase (STS) are co-localized in neurons within brain regions which are important for control of the HPA axis. Additional experiments will quantify the transfer of sulfoconjugated estrogens across the blood-brain barrier, and will test the effect of STS inhibition on the stimulation of fetal HPA function by both sulfoconjugated and unconjugated estrogens. We propose that rising concentrations of estrogens in fetal plasma at the end of gestation, themselves dependent upon increasing fetal HPA axis activity, contribute to the function of an endocrine positive feedback loop which augments both basal and stimulated fetal ACTH secretion and ultimately culminates in the birth of the fetus. The experiments proposed in this grant application will provide evidence that sulfoconjugated estrogens can be biologically active within the fetal brain and that they can be physiologically relevant in terms of fetal HPA axis control. We believe that this is highly logical, given the high activity of STS within the fetal brain and given the incomplete nature of the fetal blood-brain barrier. These experiments will provide important information which will be of critical importance for submission of an R01 application focused on STS within the fetal brain. These experiments will likely reveal an enzymatic step within the HPA-estrogen positive feedback loop which is pharmacologically manipulable. If so, the results are relevant to the control of parturition as well as the control of responses to stress.